1. Field of the Invention
The present invention relates to a chemically amplified negative photoresist composition for use in the formation of thick films, to a photoresist base material and to a method of forming bumps using the photoresist base material. More specifically, the invention relates to an alkali-developable chemically amplified negative photoresist composition for the formation of thick films, which is suitable for photofabrication such as bump formation, wiring, metal post formation, interlayer insulating film formation, circuit protective film formation and processing and manufacture of precision parts, carried out when circuit substrates are manufactured and semiconductors and electronic parts are packaged on the circuit substrates such as in chip size package (CSP).
2. Description of the Related Art
The photofabrication is a generic term for techniques in which a photosensitive resin composition is applied to the surfaces of process articles and the coating films formed are patterned by photolithography, followed by chemical etching or electrolytic etching using the patterns as masks, or electroforming chiefly using electroplating, any of which are applied alone or in combination, to fabricate various precision parts. This is prevalent in the current high-precision microfabrication techniques.
With the downsizing of electronic equipment, there is a rapid progress toward higher integration of LSIs, and a demand has been made on higher density of packaging based on multipin thin-film packaging for mounting LSIs on electronic equipment, downsizing of packaging and two-dimensional or three-dimensional packaging techniques by flip-chip system. In such high density packaging, protruded electrodes of 20 μm or more in height, called bumps serving as connecting terminals, rewiring between peripheral terminals on the wafer and the connecting terminals and very thick film wiring called as metal post must be highly precisely arranged on the substrate, and it has become more required to make the bumps higher in precision so as to be adaptable to any further miniaturization of LSIs in future.
Photoresists for the formation of thick films are used as materials for the package fabrication or IC bumping process. The term “thick films” used herein means and includes films having a thickness of from 20 to 150 μm. Using such a patterned thick film as a mask, bumps or metal posts are formed by plating process, and lead frames are formed by metal etching process. The pattered thick film is also used as a mask in dry etch process.
For example, Japanese Patent Laid-Open Nos. 10-207057, 2000-39709 and 2000-66386 disclose thick-film photosensitive resin compositions which are used for the formation of bumps or for wiring. These conventional thick-film photosensitive resin compositions require large amounts of reaction initiators in order to sufficiently react overall of the resulting resist films each having a large thickness. However, large amounts of reaction initiators may deteriorate compatibility or stability in preservation. Demands have therefore been made on reaction initiators having higher sensitivity.
Chemically amplified resists containing acid generators have been used as photosensitive resist compositions having high sensitivity. In these chemically amplified resists, a protonic acid is generated from the constitutive acid generator upon irradiation of active light or radiant ray and then induces an acid catalytic reaction with a base resin in the resin composition as a result of heat treatment after exposure. Thus, the chemically amplified resists have significantly higher sensitivity than conventional resists each having a photoreaction efficiency (a reaction per photon) of less than 1. As an example of chemically amplified negative resists, L. E. Bogan et al. disclose a resist containing a polyvinylphenol and a melamine derivative in combination in Proceeding of SPIE, 1086, 34-47 (1989). However, when thick films are formed using these chemically amplified resists, the resulting thick films invite cracking and do not exhibit required plating resistance.
To exposure such conventional photoresists for the formation of thick films (hereinafter briefly referred to as “thick-film resists” or “thick-film photoresists”), a contact exposure process, in which portions to be exposed are in contact with the resist, has been used in many cases in order to avoid diffusion of light during exposure. However, proximity exposure (gap exposure), in which the portions to be exposed are in the proximity of but not in contact with the resist, is advantageous from the viewpoint of manufacturing process. A demand has therefore been made on thick film resists that have sufficient performances even when used in the proximity exposure process.